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What fundamental differences separate low-budget airlines from traditional airlines?
Low-budget airlines have made it incredibly easy and cheap to travel far and often, and yet at first glance they seem nearly identical to traditional airlines. Sure, you have to pay for meals, but a few dollars/euros here and there shouldn't make THAT drastic a difference. So what is it that makes a budget airline a budget airline?

You're underestimating the sheer volume of fees LCCs levy on passengers compared to (most) major carriers, especially in Europe. Booking fee? Book a seat at all? Fee. Check in early? Fee. Aisle seat? Fee. Print your boarding pass at home? Fee. Priority boarding? You guessed it. Check in at the airport? Yep.

I'm only half-joking when I say that a good part of the difference in many cases is that traditional airlines have unions and billions of dollars worth of shortfalls in their employee pension plans while LCCs don't.

Well, apparently on budget airlines they allow people on who continually fart until fights break out and the flight attendents encourage it or something and the plane has to make an unscheduled landing and kick everyone off.  If that sort of thing happened on a traditional airline the flight attendents would probably not encourage the fighting.

@egid - I've travelled on several of the most prominent European budget airlines, and have never had to pay a fee for booking, for aisle seets, for pre-printed boarding passes, or on checking in.  Yes, reserving specific seats usually has a fee attached, as does hold luggage, and priority boarding of course has an associated fee (otherwise everyone would want it and therefore the system wouldn't work!), but its quite easy to travel without paying any additional fees, at least if you're willing to travel light.

There are some really good animated youtube videos that explain this in detail.

@Jules You probably haven't flown with Wow then, who charge for hand luggage (you're allowed a small item that fits under the seat for free).

Last summer I flew from San Francisco to Ireland with my two teens, by the time I had added bags for all legs (including hand baggage) the cost for a non stop, faster flight with Aer Lingus was a little over $200 more that the lowest airfare I could find (Wow) for the three of us. Given that we were already spending over $3k, this was a no-brainer. (Plus, Aer Lingus ground staff & air crew actually care about their passengers and treat them as humans. Not a comparison with Wow, with whom I have never flown.)

@Jules I'm not saying you have to pay those fees. But those are fees that often are completely absent at mainline airlines! You have the option to pay them to RyanAir or EasyJet, but many of the fees don't exist on say Lufthansa or Air France. I have definitely been given the opportunity to provide RyanAir with money for those things, and they are in fact lifted straight from RyanAir's fees page. :)

Generally speaking:

They operate a limited number of airframes if not a single one (for example Ryanair operates 420 737-800's and only a single 737-700 for chartering). This allows them to buy planes in bulk as well as cut down on spare parts reserves, pilot proficiency training in multiple airframes and overall maintenance costs by only needing to hire mechanics proficient in a single airframe. They can also easily substitute an aircraft on a given route as they are all configured the same.
By having seats that don't recline and sometimes no first class they can really pack people in. Those extra customers make a big difference. There has even been some buzz about packing more people in with "standing" seats but I have not seen anything certified on that front as of yet.
In my experience they tend to have less lenient refund and change policies. For example Spirit charges a $90 cancelation fee and most tickets are around that price.
The meals tend to be pre-packed and may have a longer shelf life than the hot food served on other airplanes. This means they can buy it in bulk (read cheap), in advance and store it, and there is no need to coordinate the delivery like there is with hot food. Cold food can hang out at the gate waiting for the plane.
There has been a big move to use mobile devices for a lot of classic functions like check in and boarding passes. This allows them to not need checking counter employees or curb side employees etc. This labor cost is a huge savings. In many cases the flight attendants may also be the  boarding/gate personnel.
Some of them also avoid major airports and may elect to fly out of smaller regional airports. This helps keep fees down and some satellite airports may be substantially less busy allowing them to get in and out faster.

As per @Jamiec's comments

Shorter turn around times

No seat back pockets so nothing to clear out.
No pillows and blankets to pick up
Cabin crew may do the majority of cleaning

As @757toga notes some of them also may keep pay low or act as a jumping off point for young pilots to get their type certs and build hours.
Related questions:
How can Ryanair offer flights for a penny?
What's the typical cost (and its breakdown) for a long-haul commercial flight?

Discount pilots? That's not scary.

@757toga not only lower pay on flight crews, but other departments as well. I've done an IT project with one LCC - their security protocols were almost nonexistent, code was buggy, etc.

Also, customer service. I've had flights cancelled both on a low budget airline and on a regular one and bloody hell I could feel the difference.

Low cost also has less nice flight times usually. They start at 5 in the morning, so they can fly 1000 km back and forth 3 or 4 times a day, instead of only twice. Since the plane and parking fees are very expensive, this reduces cost a lot.

Cabin crew may do the majority of cleaning with the rest done by the nicer passengers clearing up after themselves.  I got the last Easyjet of the day back to home base several times, and the cabin crew were getting ready to clean the cabin thoroughly -- there were no cleaners. By the time I boarded the turnarounds were under pressure so they gave it a very quick clean at the gate; I was sometimes first on as I knew roughly which gate before it was called, and they were still at it.

@rickboender Im not sure that helps keep the cost down, I was always under the impression that the early flights cater to the business market who flies to cites for a single day (for meetings etc). Many of the pilots I know that fly for the big carriers but fly regional routes also fly 3-5 hops a day. I dont think that behavior is unique to low cost operations.

They certainly do avoid certain airports. Try to travel to Frankfurt with Ryanair. You'll end up 2 hours by bus away from the actual city.

For a true LCC (Low Cost Carrier), the answer is simple :

Single-line buisness model
Volumes
Cost & efficiency discipline

Those three items are what, fundamentally, makes an LCC an LCC.
I will briefly explain :

Single-line business model

A true LCC will define its business model as "seeking to offer its customers the lowest fare possible for a flight".
That is ultimately the goal that underlies everything else that goes on in the LCC's business.  That is their "mission" if you like that word !

Volumes

In practical terms, the biggest element in the LCC's "mission" to minimise fares is to maximise the numebr of passengers on their aircraft.  The industy terminology for this is ASK (Available Seat Kilometers).  LCC airlines will have some of the highest ASK figures in the industry.
Part of the reason why the seats on LCCs are so "primitive" is to minimise the risk of a seats being taken out of service for technical reasons, hence impacting negatively on ASK.
Of course empty seats are worthless. So alongside high ASK figures, the LCCs will also have some of the highest load factor (i.e bums on seats) figures in the industry.  Or in other words, they will have some of the highest ASK yields in the industry.

Cost & efficiency discipline

Closely associated with the ASK figure is the CASK (Cost Per Available Seat Kilometer - excluding fuel and depreciation).  Again, the LCC carrier's "mission" is to minimise CASK.
The LCCs achieve low CASK figures through aggressive cost and efficiency discipline.
EVERYTHING, every single tiny little thing you could ever think of (both "tangible" e.g. toilet paper and "intangible" e.g. the "cost" of someone taking a lunch break) in the business will be assigned a cost and there will be a constant and aggressive drive to keep lowering those costs. Management reporting will be heavily skewed towards costs.
One source of "excessive" costs is of course poor efficiency. Therefore alongside an aggressive cost control programme, there will be an efficiency programme running in parallel.  One key component is finding ways to minimise time on the ground (if a aircraft is on the ground its not making money).  But there are also things like making sure routes are efficient (so they can also minimise time in the air so they can pack more flights into the day).
Its fascinating but very complex topic. Far too complex for a StackExchange answer !

One of the largest differences is luggage. Originally, checked luggage was part of your ticket, but airlines have been steadily cutting those back, largely because the costs associated with handling luggage

Less work for the ground crew

Fewer complaints (lost luggage is a common traveler complaint)

Less overhead in compensation over luggage

Collins waited for his missing bags. And waited. Finally, he called Delta out on social media and connected with a customer-service representative who helped him track down the bags three days after they’d gone missing. Eventually, the airline cut him a check for $230, the amount of money he and his wife had to spend on clothes and toiletries.

While almost all major airlines charge for checked baggage (Southwest is still included for now), the cheaper airlines also make you pay for carry-on bags. If you've flown at all recently on a regular carrier, you know that almost everyone now tries to cram it all into carry-ons since they are still free. This creates all sorts of hassles in boarding as, once the overhead space is full, you now have to start checking those carry-ons (I invested in a smaller carry-on to avoid that problem).
The cheap airlines hate carry-ons for this reason. So much so, they actually charge you more for a carry-on than a checked bag (as of this writing, both US cheap carriers, Spirit and Frontier, are \$35 for a carry-on and $30 for a checked bag, provided you pay at online booking). Personal items (a large handbag/purse or backpack) are still free.

"While almost all major airlines charge for checked baggage" -- AFAIK this is not the norm outside North America...

Afaict for flights within Europe the most basic economy ticket (aka what will show up by default on the price comparison sites) often does not include checked baggage while for intercontinental flights it nearly always does.

@DanMašek It is the norm for regional flights outside North America, at least in my experience. Air France/KLM does. Alitalia does. TAP does. Philippine Airlines does. Lufthansa/Swiss does. It looks like maybe British Airways is an exception. Intercontinental flights (on legacy carriers) typically include luggage for both North American airlines and those based elsewhere. Intercontinental flights on LCCs typically don't include luggage (and make a killing on bag fees.)

Interesting. I've noticed that for example Lufthansa now has a "light" category that doesn't include checked in baggage, but I didn't realize it spread so fast. IMHO the opportunity to do that is useful, but being forced to do that seems, well far from ideal. TBH I've gotten kinda picky on which airlines I fly with and airports I go through, so thanks for enlightening me (as depressing as it may be) :)

@DanMašek I went looking and, sure enough, some international flights still involve free bags on carriers that charge for them domestically (i.e. Delta). Even within that category, there's an emerging "Non-bag" tier. By contrast, almost nobody charges bag fees for higher tiers like First/Business class. Since low cost carriers don't have class tiers like that, everyone is in the "non-bag" tier.

Isn't full-service airlines charging for checked luggage actually a relatively recent response to the LCCs? (Last 10-15 years, say.)

Target Demographic
The target demographics of legacy (or full-service or whatever name you prefer for non-LCC) carriers is different from that of low-cost carriers (LCCs.) LCCs target primarily infrequent leisure travelers who choose flights primarily based on lowest price, even if the flights are less convenient and less reliable. By contrast, legacy carriers cater more to business travelers who are typically willing to pay more for a higher level service and a higher degree of reliably getting the passenger to their final destination in the quickest manner possible.
Interlining Agreements, Codeshares, and Alliances
Most legacy carriers are part of one of the major airline alliances, namely, oneworld, Star Alliance, or SkyTeam. Airlines within an alliance will generally have interlining agreements and often codeshare agreements with each other. Additionally, legacy airlines often have separate interlining or even codeshare agreements with other individual airlines outside of their alliance, those these bonds will typically not be as strong as those within an alliance.
Selling Single Itineraries
Interlining agreements allow the member airlines to sell tickets that include connections to or from other member airlines as part of the same itinerary. By selling the entire itinerary as a single ticket, the airlines then take on the liability of ensuring the passenger gets from their origin to their final destination. For example, with separate tickets, if a passenger takes airline X from A to B and airline Y from B to C, airline X is only responsible for eventually getting the passenger to B and airline Y is only responsible for getting the passenger from B to C if they show up on time at B. On the other hand, if airline X sells the passenger a single itinerary from A to C via B, even if the flight from B to C is operated by a partner airline, the airline assumes the responsibility for eventually getting the passenger all the way from A to C, even if the connection at B is missed due to a delayed flight or similar.
Assuming this liability costs the airline money. Therefore, LCCs typically don't do it. LCC flight cancelled? You're likely stuck until their next flight (which may be a day or three later.) Only in extreme cases (e.g. the airport is closed for an extended period) would you have delays that severe on a legacy carrier, since they'd usually have one of their own flights or a interlining partner flight they could transfer you to.
Through-Checking Baggage
Interlining agreements between airlines, especially those within an alliance, also often include allowing passengers to through-check their bags across multiple carriers instead of needing to re-check at connections. This makes life much easier for the passengers, especially the business travelers who need to frequently travel from one region of the world to another. However, working out the necessary organization between airlines to do this costs money, both in getting their IT systems to play nice with each other and in getting their ground crews to efficiently hand baggage off to each other.
Transfers in Irregular Operations
Interlining agreements will also frequently include provisions for the airlines within the agreement to transfer their passengers to each other's flights in the event of irregular operations in order to help get them to their destination as quickly as possible.
Codeshares
In addition to the above interlining agreements, legacy carriers also often enter into codeshare agreements with each other. These agreements allow one airline to actually market another airline's flights with their own flight numbers. For example, Delta can sell seats on Air France flights using a DL flight number and vice versa. The airlines will then work out as part of this agreement how they'll split the revenue.
Loyalty Programs
Perhaps one of the biggest differences between legacy carriers and LCCs is that legacy carriers typically have extensive loyalty programs designed to entice their highest-profit customers to keep their flying within that airline and its partners rather than shopping around for the cheapest flight for each trip. LCCs, by contrast, typically have little-to-no loyalty program. The award tickets and other various perks offered to frequent fliers by these programs can represent a quite large cost to airlines, but they ultimately deem them worthwhile expenses in order to attract and keep their most profitable passengers. Since LCCs typically cater to people who aren't willing to pay extra for these benefits, they cut these costs and don't provide the benefits.
Classes of Service
LCCs typically offer economy-only configurations or at least relatively fewer and less extravagant premium cabins. Legacy carriers, on the other hand, typically have multi-class cabins. These range from wider seats with more legroom in regional first class cabins to seats the fold out into 6-and-a-half-foot-long flat beds in long-haul business class to extravagant apartments in the sky complete with dedicated beds, separate living rooms, and showers in long-haul first class. Again, this comes down to a difference in target demographic. Business travelers who need to get to the other side of the planet and arrive well-rested for an important meeting are going to be more willing to pay for those highly-profitable premium-cabin seats, while the infrequent leisure travelers targeted by LCCs won't be.
Route Networks
Another major difference between LCCs and legacy carriers is that legacy carriers usually have much more extensive route networks. Legacy carriers are more willing to provide service to less-highly-trafficked airports in order to connect those passengers to their more profitable routes elsewhere and in order to allow business travelers to reach more remote areas. By contrast, LCCs normally only (or at least primarily) run routes with high demand, especially high leisure demand. The LCC model is to be able to fill every seat on every flight of a 150+ passenger aircraft. Meanwhile, the legacy carriers are often content to fly half-full 50-seat regional jets to the middle of nowhere if it can connect passengers to their hubs for more profitable onward connections.


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What is it called when an airplane has to circle because it can't land?
Sometimes, a plane is required to circle around an airport repeatedly because for whatever reason, it is not able or permitted to land just yet.
This state in which a plane is stuck in the air in this way has a name, what is it?

It's called annoying

Fwiw, the word "circle", or "circling",  in aviation, generally means something specific to instrument approach procedures, where an aircraft is executing an approach that terminates on a runway other than the intended landing runway. Better to just use the word "Waiting" - or, even better "Holding" which is actually the answer to your question.  And Holding procedures are tightly defined, and they are generally not circles but oval racetrack patterns.

I've known it be called "thermalling"; in this context, with a few failings in airmanship!

Back when I was writing software for UAVs we called this loitering but that might have just been us bit pushers using that terminology.

@Ukko Loitering is travelling (usually orbiting) at the best-endurance speed, to maximise how long you can stay there for. It's not specific to waiting to land, and you might not even be loitering in the situation described by the question (you might be burning fuel to reduce landing weight).

@DanHulme good to know!  It makes sense, because we always tried to maximize the efficiency and deliver the most science for the buck, that loitering would be the mode we cared about.  In any emergency we always had a person take over, this was 20 years ago.  (Ironically the only crash that happened was under manual control )

It's called a holding pattern.

A "holding pattern" is a very specific type of an oval course flying over a specified fix in IFR.

Wikipedia says "The primary use of a holding [pattern] is delaying aircraft that have arrived at their destination but cannot land yet because of traffic congestion, poor weather, or runway unavailability (for instance, during snow removal or emergencies)" which is close enough for government work. It is sometimes called "the stack".

Pack 'em, stack 'em, and rack 'em.

I agree with @TimLymington that using this term this way is common in layperson discussion and media (often used metaphorically), but I think this answer would be improved by noting the technical definition and how aircraft may well stay in the air waiting for an opportunity to land without using the specific pattern known as a holding pattern.

As an alternative to saying it's in a holding pattern, we can simply say it's holding or on hold.  That's the verb; even if we argue that a a holding pattern is a specific course used for aircraft which are held waiting for clearance.

There is no generic name in aviation describing the state of an aircraft being hold up and unable to land. The simplest term I have in mind is "circling the airport".
Depending on the way the aircraft is circling the airspace, specific names can be used. Note that these terms carry specific technical meaning in aviation, although they may be misused by journalists in news articles.

Holding Pattern is an race-course pattern flown over a specific radio station or waypoint at a constant altitude. The outbound and inbound legs are 60 seconds and the turns are executed at Standard Rate Turn. A Holding Pattern is flown under IFR.
Traffic Pattern is a rectangular pattern flown at a low altitude around a runway. It consists of "upwind", "crosswind", "downwind" and "base" legs. A Traffic Pattern is flown under VFR.
360 is a circular pattern in which the aircraft maintains a constant rate of turn. 360s are usually flown by small aircraft but rarely large airliners because small aircraft can maintain a small turn radius.

It may also be none of those: the controller may just issue heading instructions to direct the aircraft around terrain and other traffic as necessary.

Not sure why a 360 would need a small turn radius. They're regularly done by airliners for spacing on approach.

@Sanchises I'd say the opposite. Usually if a small GA gets in the way of a large airliner landing on the same runway, the GA does a 360 and joins back behind the airliner. If an airliner does a 360 it may stray into departing traffic / traffic landing on parallel.

I didn't mean on final, but say 50nm out, for last-minute spacing issues after the holding fix but before the final approach, for example near Heathrow or Schiphol it's a common occurrence.

@Sanchises but then, OP said "circle around an airport", so I guess 50nm out would not count (-;

@kevin I think the OP just didn't realize that it doesn't actually happen over the airport itself. When you're holding for traffic in an airliner as a passenger, it's easy to forget that 10-15 minutes of flying time can actually be quite a long way away from the airport. For example, I've seen stacks of inbound aircraft holding for ATL nearly 100 miles away near Chattanooga when the ATL runways were shut down due to thunderstorms in the vicinity.

Also, does 'traffic pattern' really belong here? Do people actually fly the traffic pattern to waste time when they're not permitted to land, yet?

@reirab well depends on whether we are talking small or large aircraft. On a small GA airport, if the runway is blocked but the issue is expected to be resolved in a few minutes (e.g. a deer crosses the runway), why not?

Might be worth mentioning that a 360 is commonly called an orbit in the UK (and the ICAO)

@Roman my understanding was that an orbit could also be a race track shape (basing this on NATO messages for an orbit rather than any flight xp)

Busy airports may have an established process for queuing arrivals so they can be spaced efficiently for landing.  In the UK these are known as "stacks" or "holding stacks", and aircraft in them are said to be in a "holding pattern".  See the Heathrow Airport website for examples of this phraseology.
Smaller airfields are unlikely to have designated stacks, but may advise aircraft to "orbit".  This advises the aircraft to fly in circles either a specific number of times, or until further instruction is received.  For example "G-ABCD, for spacing orbit once".  See the CAA Radiotelephony Manual for official guidance as to the use of this term.   An aircraft doing this could be said to be orbiting.

That’s a VFR traffic pattern.  It’s a type of pattern which allows a fixed wing aircraft to transition from an arrival to sequencing for an approach to landing under visual flight rules.  Airplanes practicing takeoffs and landings will enter and exit the pattern from the runway using the departure and final approach legs.  Aircraft can also continue to hold over the airport in the traffic pattern for traffic separation, right of way, etc.
Other possibilities could be holding patterns, though these are generally not flown directly over an airfield, marshaling patterns or simply an aircraft practicing ground maneuvers over a non-towered airport, though this practice is frowned upon for safety reasons.

5 downvotes and not a single comment?  What is wrong with you people?

@Phil I'm not one of the down voters but from experience, it'd be futile on this answer.

Can anyone here explain exactly what’s wrong with the answer?

I would not call the VFR traffic pattern a state of "stuck in the air and cannot land" which the question is asking about.

I’m not sure there is a state where an airplane is “stuck in the air and cannot land”.  I’d say that it’s an observation of a VFR traffic pattern or similar marshaling or holding pattern over the airport and the author thinks that’s what’s going on.

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Why are emergency water landings the majority of the pre-flight safety briefing if they're so rare?
According to an article published in the New York Daily News a few months after the "Miracle on the Hudson", when US Airways flight 1549 safely landed (watered?) on the Hudson River to the survival of all 155 souls on board, this incident was only the second ever emergency water landing that did not result in significant injuries and casualties in commercial aviation history.
And yet, the safety briefing at the beginning of every commercial flight adhering to aviation regulations spends more time discussing water landings than nearly any other topic, focussing on how to use life jackets, where to find them, where to inflate and where not to inflate them, how to use the emergency rafts, and so on.
Further, a very large amount of weight on commercial aircraft is devoted to equipment chiefly or entirely present to aid in water landings, with most commercial aircraft today having four or six emergency life rafts, hundreds of life jackets, and even the design of the aircraft itself adhering to minimum float time requirements.
With water landings so rarely attempted and even more rarely successful, why is there so much emphasis put on the current, ineffective methods? In other words, if water landings with current methods and technology generally don't work, why hasn't this changed?

No modern aircraft has "four or six" dedicated emergency life rafts. The escape slides also serve as rafts. At most there is one additional raft and that's usually found on aircraft that make you evacuate off the wing without a slide (737, MD-80). Further, if you want an aircraft to be able to be pressurized, it will naturally float. There's no weight lost in that.

Because, as has been practically demonstrated (see the article you mention) water landings can be quite survivable if procedures are followed, whereas most other crashes aren't.

I would say that more time is spent on water landings because there's more for you to deal with during a water landing.  A non-water landing basically involves unbuckling your seatbelt and getting out an exit (leaving your luggage behind). Water landings add the need for life vests (where are they, how to put them on, inflate them--don't do that until outside or you may not get out), and rafts.  More stuff to talk about takes longer.

The only thing you can do to increase your survival likelihood in a non water landing is to scream loudly and/or have a chat with your preferred deity(ies.) In the case of a water landing - there are things you can do in addition to the afore mentioned.  Thus the briefing.

@user71659: According to this safety card, at least some 737-800s do have four dedicated emergency life rafts.

@MikeBrass I thought the chat with your preferred deity was to make your peace with your fate, and as for screaming loudly... there's a certain humor to the idea that people scream because they believe it will increase the likelihood of survival of the plane crashing.

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How does bypass air work?
As far as I’m aware, high-bypass engines have far higher fuel efficiency than low-bypass engines, and as a result the effective specific impulse of the engine is increased. What I’m trying to figure out, and what no previous question I’ve found asks, is where’s the tradeoff?
There’s no free lunch, and a massive specific impulse boost for nothing doesn’t seem possible. What are you giving up with high-bypass engines?

Your first link is a question about the history of bypass engines, and your second is about the progression of bypass engines over history and into the future, though some of the background provided is certainly helpful. I’m not sure I see the connection to my question about engineering tradeoffs

The bypass lowers the exhaust velocity of the engine. Which improves the efficiency, but limits the flight speed.
In jets (both turbojets and turbofans) the exhaust velocity does not grow with the inlet velocity. That means that with increasing flight speed the delta-V decreases, which increases propulsive efficiency, but the engine obviously can't produce any thrust when delta-V reaches zero, so this places an absolute limit on how fast the aircraft can fly.
Because efficiency is best close to the maximum speed, higher bypass is better for flying slower (airliners fly M0.8 to M0.85), while flying faster requires lower bypass (fighters tend to have only 0.5:1 to 1:1).
The other thing is that the core needs to be able to produce the power to drive the fan. So the bypass ratio raises as the designers learn to build stronger and more efficient turbines and compressors that allow higher compression ratio.

In addition to the good answer given by @JanHudec I can add a few things.  To have the same thrust with a high bypass, the engine has to be a larger diameter, which has a bunch of downsides.  This tends toward more weight, longer fan-blades, more gyroscopic forces etc.  It can often lead to longer landing gear except on the 737, where instead the bottom of the nacelle was flattened.   The engine-out drag on a high bypass would also tend to be higher.

In a conventional turbojet engine, all the air used to produce thrust by accelerating it flows through the core of the engine, where the combustor cans are. All of that air gets compressed and shot through the whole engine at high speed- but only a small part of it takes part in the combustion process. High speed means lots of frictional losses.
Bypass air (which has been accelerated only by the first-stage fan) does not flow through the (high speed) core and so dissipates less energy as friction, as pointed out by Jan Hudec. Here is a way to think about this in simpler terms:
Imagine a 100 horsepower O-200 engine as used in a Cessna 150. Its output drives a propeller, through which a significant mass flow of air is accelerated upon traversing the propeller disc, generating thrust. The power used to drive this "fan" is supplied by a relatively tiny mass flow of air through the engine which gets sucked in through the induction system, mixed with fuel, compressed, ignited, expanded, and exhausted. The pumping losses associated with this mass flow are small relative to the thrust developed by the fan- a very efficient arrangement.
So you can think of a piston engine driving a propeller as an ultra-ultra-ultra-high bypass ratio engine!

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Single-engine plane with long range flyable in Europe
Not 100% sure if choice of airplane is on-topic here, please let me know if not.
Just got my PPL, and looking for a choice of single-engine meeting a few conditions:

Long-range, definitely >800nm, ideally >1200nm
Can be flown IFR with only IR (no special type/class ratings) under both FAA and EASA
Available used for under $100K, ideally less

Are there planes out there that meet these conditions?

There are definitely aircraft which meet some of your criteria, namely range above 800nm and can be flown IFR. I'm not going to list them all, notable examples would be the Mooney M20 series, the Cirrus SR22, Cessna 182 and Cessna 210. No doubt there are more. The issue will be cost, you can find older M20s and Cessnas for your budget, the cheaper they are the shorter time the engines have until they need a costly overhaul and the more work they'll need.

I'd generally give it more time on accepting an answer, but this one really checked all the boxes. Some classic Cessnas, the Mooney (though they just laid off all their staff, so remains to be seen whether they'll be maintainable), and the SR22, which might be a bit young to be in the right price range, but does fantastically on range, punch, and gotta love that parachute. Hard to argue with these choices right here :)

I'm glad you think it's useful @TheEnvironmentalist. Buying an airplane is one thing, maintaining it is another thing cost-wise. Unless you are flying a lot buying a share may be a better choice.

I imagine that owing a <$100k aircraft is like owning a Mercedes or BMW from the early 2000s - the price of entry is low (relatively speaking), but oh the maintenance costs!!

I have one of each of those brands and they have been some of the cheapest cars to maintain @FreeMan. They just last. Well, maybe the M3 is a special case, but it's an M3!

Not wishing any ill upon you, @GdD, but I'm guessing you haven't had an electronics failure yet. Or, maybe it was from the 90s when all the electronic gee-gaws started coming in, but they weren't all that reliable yet.

Both mine are from the early 2000s @FreeMan. I had a Porsche 944 as well which was remarkably inexpensive for parts. Sadly written off when someone crashed into it in front of my house. I miss that car.


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Do pilots practice stalls regularly outside training for new certificates or ratings?
I know that stall practice in real aircraft is part of PPL training, and that airline pilots training for type ratings practice stalls in simulators. After obtaining a certificate/type rating, do pilots continue to practice stalls regularly?

That entirely depends on the pilot. I practice stalls, steep turns and practiced forced landings on a regular basis, partly to keep my skills up and partly because I find it fun. Some pilots never do these things except when they have to in recurrent training and checkrides. For private pilots there's no requirement to practice these regularly.

Ancedotally, glider pilots often practise stalls (as well as things like steep thermalling turns) on flights where there is no chance of lift and you may as well do something useful/fun with your time in the air.


text:
Does the FAA ever lose in court?
It seems that FAA legal interpretations are often cited as definitive evidence that something is legal or illegal. In the case of using them to show that something is legal, that makes sense, but I don't get how they can prove something is illegal. Wouldn't ant enforcement be an adversarial court case such as FAA v. Pilot? Isn't there a chance the court could rule that the FAA is wrong and whatever the pilot did is actually legal?

When a pilot disagrees with a FAA enforcement action, they bring their case to the NTSB administrative law judges to decide which party is correct.  You can read about these cases here.
https://www.ntsb.gov/legal/alj/Pages/ONOQuery.aspx
Yes.  The FAA has lost some of these cases.


text:
What is a "cell phone parking lot"?
This question about sirens refers to a "cell phone parking lot." I know use of cell phones is prohibited on planes; is this true in most of the airport as well?

This belongs on Travel.SE.


text:
Why do acronyms aviation often have words in orders unusual?

"Airplane Single Engine Land" (rather than "Single Engine Land Airplane"
"Certified Flight Instructor Instrument" (rather than "Certified Instrument Flight Instructor)
"Medical Certificate Third Class" (rather than "Third Class Medical Certificate"; although AFAICT "Medical Certificate Third Class" is generally not used except as the title of the document itself)

It seems like aviation has a lot of acronyms with the words in orders unusual. Why?

It's a cataloging practice (raised to an art form by the military) to place the root (or common) noun, (or root noun cluster) first, with variable adjectives following, in order to facilitate indexing. For example, all the medical categories can be listed under M in an index, and all the airplane categories can be indexed under A.  Way easier to find things.

Thank you! So it's basically for the same reason that book indexes often use phrases like "Gettysburg, Battle of"?

Exactly.  You could probably call it a universal publishing practice.

They make more sense if you insert commas after each word.  Or, say them in a Yoda voice.

Comedian, you are. Go for laughs, you do.

@TypeIA I made an edit to accommodate your comment.


text:
Has FDR data from a previous flight ever been useful in an accident investigation?
Has there ever been an accident investigation in which FDR data from a prior, seemingly uneventful flight was useful, for example by showing that a problem had begun to develop prior to the accident but had not been noticed?

@mins that sounds like an answer?

I did a quick search online, it confirms previous flight recordings have been used and a link with the accident evidenced. Investigation agencies obviously try to make the most of the material they can get, I don't know how often looking at previous recordings brings additional elements.
Two random examples:

An aircraft accident in Russia, in 2013, investigated by Russian IAC, and

the Germanwings accident in 2015, investigated by French BEA.

VQ-BBN in 2013, approach at Kazan International Airport (UWKD), Russia
Boeing 737-53A operated by Tatarstan Airlines. ASN:

The need for a go-around was caused by the position of the aircraft relative to the runway, which was the result of "a map shift" effect (Map shift, an error in the determination of the aircraft position by onboard systems) by about 4 km, the crew's inability in the circumstances to integrated piloting and maintenance of navigation with the required accuracy, and the lack of active assistance of the air traffic control service under the long-term monitoring of significant deviations from the approach procedure.

The recordings of older flights were used to show the repeated map shift and the offset trajectory of the aircraft at multiple times for the the same approach. The report includes plotting of related approach ground tracks.
From the investigation report of the Interstate Aviation Committee (IAC):

The analysis of previous PIC flights FDR record (Section 1.18.8) revealed that ...

during the previous approaches there were instances when IRS current position errors reached significant values that could be compared with the error remarked in the accident flight. The start of the base turn during all approaches was matched the approach pattern with an acceptable accuracy.
in all cases when there was significant deviation of the actual flight path from that calculated by the IRS, the crew deactivated L-NAV mode well in advance and entered the base turn point with HDG SEL mode engaged.
during all approaches the crew engaged the VOR/LOC in due time, which ensured the aircraft intercepted the localizer in automatic mode.

IAC concluded the problem with the map was not new, there was no maintenance initiated.
D-AIPX in 2015, Pilot suicide while overflying French Alps
Airbus A320-211 operated by Germanwings (ASN)
From the investigation report:

All of the data from the previous flight, from Düsseldorf to Barcelona, was recorded on the FDR. The recordings from the CVR included the last 50 minutes of this flight.

The BEA noted what can be interpreted as a attempt to crash the plane in the previous flight. This happened, like for the actual accident, while the captain had left the cockpit.

The autopilot altitude was set briefly, at multiple times, to 100 ft. The BEA also tried to exploit the QAR (the non-protected data recorder used by the maintenance team), but it was too damaged for retrieving data.


text:
Where do you store an airplane for an overnight stop?
When a private pilot flies a plane like a Cessna 172 on a two-or-more-day trip, where do they store the plane during overnight stops?

Reminds me of a dad joke: Q: Where does an airplane sleep?  A: AirBnb  -- I'll be here all week... please tip your waitresses kindly.

Most small airports have places for transient aircraft parking.  Sometimes they are paved and have tiedown spots, other times is might be a grass field.  Prepared pilots may bring their own tie down anchors if windy conditions are anticipated.
Larger airports often have Fixed Base Operators (FBOs) who will host you, providing tiedowns, maybe hangar space, fuel, snacks, a lounge, areas for flight planning, etc.  Sometimes a courtesy car is even available.

Some FBOs have hangars and, for an additional charge, will let transients park there overnight.

I'm sure that, for the right price, one could find hangar space just about anywhere that has hangars. Probably wouldn't be worth that "right price", though.

Usually on the apron.  e.g.: See Apron parking fees (for my local airport)

It never hurts to ask if the FBO has a hangar available for overnight rent! As an example, on my first solo cross country, the return flight had to be delayed 2 days after a dying battery delayed departure to the point unfavorable weather conditions began to develop. With thunderstorms forming, I paid $10/night for a hangar at KRLD - very cheap insurance compared to hail damage.

I do a lot of business trips and I always follow the same pattern...  (a) Check out the airport ramp layout on google  maps aerial images.  (b) Look in the AFD (via Foreflight in my case) for FBO info.  (c) Take a look at Skyvector or AirNav to see if FBOs are listed.  Then I usually call.  I've actually found that about 1/5 of my stops are at locations where you just pay a tiedown fee, but before they go home at night, they'll pull your plane in their hangar if the weather is threatening, and only charge you the hangar fee for the nights they do.  (Discussed in advance, obviously.)

I have amended my answer based on several comments to show hangar space more likely available than I insinuated.  I haven't done a whole lot of GA overnights and thought hangaring overnight was a rare, expensive, and usually unnecessary luxury.


text:
What happens if you put car gas in an airplane?
If I understand correctly, small planes like the Cessna 172 use 100 octane leaded gasoline with no ethanol. What would happen if you put regular unleaded car gas with ethanol in one?

It depends. If the anti-knock rating is too low for the engine conditions then you will get detonation, which both limits power and causes major internal engine damage. ("octane" is a common term for one anti-knock rating system.)
Ethanol is highly polar and causes problems with moisture absorption that then turns into either ice crystals in the fuel lines or separation of the fuel in the tank into an aqueous layer, that will outright stop the engine, and a very low octane petroleum layer even worse for detonation than the fresh fuel. Water isn't attracted to 100LL and will settle out for easy removal without changing the chemistry of the fuel.
There is also a difference in the vapor pressure of the two fuels that can create bubbles in the fuel system and starve the engine.(Altitude, temperature, and fuel pressure dependent issues.)
Longer term. The seals and materials of the airplane fuel system may or may not be a material compatible with the ethanol formula.
Ethanol is added as an oxygen carrier, along with the moisture problems this oxygenation greatly reduces storage life to only a few months in most climates and causes gums and gunk formation during storage; 100LL has anti-oxidants added and has a shelf life of a couple of years in storage. Ethanol is also highly correlated with metal corrosion of fuel systems parts during storage.
The small amount of lead in 100LL is needed to prevent damage to the exhaust valves of some engines. (Based on the alloys they used) This is due to micro-welding under heavy load [hot valves] which results in accelerated erosion of the valve seats, lead makes a protective coating to prevent this. This same coating will damage sensors and catalytic converters in many engines designed for unleaded fuel. The older high-lead green 100/130 fuel also used lead as an effective primary anti-knock ingredient.(allowing a cheaper base fuel)
The actual anti-knock rating needed depends on several factors, some C-172 used engines rated for 80/87 red avgas and they would likely be ok with the antiknock rating of highway ethanol 92, but others were made for or upgraded for 100/130 or 100LL which is approximately 105-110 on the highway anti-knock scale. (All USA rating systems, Europe is different.) And of course, you still have all those other hazards with ethanol-gasoline formulations. Pure ethanol is a different kettle of fish completely, the above info is for blends of petroleum and ethanol.

First, it's important to note that the octane rating for aviation gas has nothing to do with the octane rating for car gas. This is because octane isn't calculated, it's measured. A specialized piston engine is run on the gas, and the compression ratio is increased until knocking occurs. The compression ratio is compared to a reference fuel, and that's the octane number. Because the Standard Test Engine is different for avgas compared to mogas, the octane number for those fuels isn't directly comparable.* It is generally true that a higher octane in an automotive context corresponds to a higher octane in an aviation context, but even that's not guaranteed.
Because of this, you can't know how well the fuel will resist knocking and pre-ignition when in an aviation engine. If the fuel just happens to have a high-enough anti-knock rating, it will most likely run just fine. But if it doesn't, then all the problems associated with knocking will occur, including damage to the pistons, piston rings, head gaskets, and even eventual destruction of the entire engine.
*Imagine trying to assign a single score to a bunch of runners, to decide which one is "best", based on a single test. Well, some people have good endurance without having a high top speed, and so would be good in a marathon but not at sprinting. So, the score those people would have is based on whether you run your test with a sprint or a marathon.

It depends on the engine type and the fuel the engine manufacturer has designed it for.
In the Cessna 172S PIM, it says:

WARNING
Use of unapproved fuels may result in damage to the engine and fuel system components, resulting in possible engine failure

100 LL (Blue) and 100 (Green) are both approved for use on the 172. 100 fuel is used on some higher performance cars, so yes, the 172 can use "car gas", but only a specific type.
Now, if you use the "standard" car gas that you find at the gas station, you would most likely get detonation which is when the fuel-air mixture in the cylinder explodes instead of combusting normally. This is because the lower octane fuels are lighter molecules and vapourize a little more easily than higher octane fuels will. Because they vapourize more easily, they will combust/detonate much faster and possibly earlier than the higher octane fuel will.
There are also other engines that are designed to use diesel such as the Austro E-4 and Continental CD-230.

What is 100 (Green)? Is that unleaded?

100-octane aviation is leaded gas. It's dyed green to keep it separated from 100-octane low-lead (100LL), which is dyed blue. Unleaded 100-octane would be 100UL.

@HiddenWindshield oh okay. So 100 has the most lead, 100LL has less, and 100UL has none?

@Someone Correct.

@Someone note that unleaded avgas, 100UL, was just approved I think less then a month ago. The company that defined the mixture now has to test it with all the various engine types and provide supplemental type certificates and IIUC you'll have to buy the one for your engine to be legally allowed to use the 100UL fuel … when it's even available on your airfield.

@JanHudec so the company says "this fuel is safe to use in XYZ engine," but then pilots have to buy a copy of the certificate?

@Someone that's my understanding of it. Because someone has to pay for all the testing to ensure it is indeed safe to use with that engine.


text:
Is it feasible to make 2 meter amateur radio contacts from a small GA plane?
Is it feasible to make 2 meter amateur radio contacts from a small GA plane (e.g. a Cessna 172)? If so, what equipment is required?
Most of the stations being contacted will be using vertically polarized antennas, which are not ideal for up/down communications. Does the normal recommendation to orient your antenna the same as those of the stations you want to contact apply in this case, or would it be better to use a horizontal antenna on the plane? (Assume it is not feasible to have the stations on the ground use horizontal antennas.)
I'm specifically asking about technical feasibility, not legality.

It's likely you're taking about a plane at 3000-6000ft, which isn't terribly vertical- compared to ISS or a satellite, at least.

@tedder42 for satellite contacts, horizontal antennas are used. People who work satellites usually have separate antennas.

from a small GA plane or for a small GA plane?

Isn't this more about amateur radio than about aviation? Perhaps we should move this to ham.stack?

@DeltaLima I wasn't sure which site to ask on; feel free to move it if you want.

@Someone, I leave this up to the community to decide. So far there are tow answers and nobody suggested to close the question, so the conclusion is that it fits here.

You can certainly use a 2 meter handheld in a plane to boink a repeater in your vicinity. You can do this with the rubber ducky antenna attached to the handheld or you can install a second dedicated VHF comm antenna on the belly of the plane to connect to the handheld, since the 2 meter band sits right at the top of the VHF comm frequency band and will match up fairly well to the handheld.
Since the 2M band is slightly shorter in wavelength than the VHF comm band, the comm antenna will be slightly too long for 2M- meaning that it can easily be shortened or  "pruned" to length to obtain a near-perfect match.

Thank you! How much do polarization issues affect this?

repeaters use vertical polarization, point-to-point 2 meter is horizontal. I have no trouble hitting the repeater 20 miles from me on my handheld even when I am holding the radio sideways.

Thank you! Where are you located? I thought all FM activity was normally vertical and SSB/CW was horizontal, but it could vary by country or region.

Oregon, USA. My 2 meter repeater-boinker yagi is indeed vertically polarized (elements are vertical) and to use it for 2M SSB requires turning it horizontal.

Oh okay, I'm in Oregon too. So for FM simplex would also be vertical, right?

Corvallis maybe??? I don't do FM simplex so I do not know.

The aviation VHF comm antennas are vertically polarized. VHF nav antennas (VOR/LOC) are horizontally polarized.

@Gerry, most excellent!

Okay. AFAIK generally all 2m FM is vertical. But considering that air-band comm antennas are vertical, I can't imagine it being an issue.

Two meter HTs work fine from general aviation aircraft. I used mine in the early 1980s with autopatch to make a telephone call to a friend to meet me at my arrival - before cellular phones were common.
In the 1970s, a USAF Major was piloting a Lockheed C130 Hercules when it suffered electrical failure which disabled its radios. He pulled his HT out of his flight bag and called the destination air base control tower to get approach and landing clearance. The tower was both surprised and impressed (I attended USAF engineering class with him in 1975).

Thank you! How did he contact the tower with an HT? Was it an air band radio?

Auto patch, which many repeaters have, allows a user to make telephone calls through the repeater.


text:
How do ETSOAs/TSOAs expire?
EASA can issue an ETSOA (Euro-Technical Standard Order Authorization/Approval) for certain equipment design, to simplify installation authorization on an aircraft type design.
I found no notice of such an ETSOA expiring at a certain date or for a certain certification basis. Still, at some point regulations evolve and ETSOA'd equipment may stop meeting those regulations. How is this managed?
As an example: suppose a few years ago, an equipment was designed  and manufactured as per some ETSO, and approved by EASA for that ETSO under a certification basis including the now obsolete DO-178B.
Now or in the near future, an aircraft manufacturer wants to use that equipment on a new aircraft type that is required to use the updated DO-178C. What would the aircraft and the equipment manufacturers have to deal with as extra certification work for that equipment? And in the case there is no extra work, how will the EASA deal with new regulations in the years to come?
(this question is not specifically targeted at EASA's ETSOAs: consider the same question for FAA's TSOAs and similar mechanisms in other countries)

Just to note, DO-178B isn't "obsolete" for products that used it during their certification. New products must use DO-178C but there isn't any restriction on previously TSO'd equipment. I'm sure there is still quite a lot of equipment flying that was certified with DO-178A.

Also the last paragraph is really another question, you might move it to a new question

From the Code of Federal Regulations CFR 14

§21.613   Duration.
(a) A TSO authorization or letter of TSO design approval is effective until surrendered, withdrawn, or otherwise terminated by the FAA.
(b) If a TSO is revised or canceled, the holder of an affected FAA letter of acceptance of a statement of conformance, TSO authorization, or letter of TSO design approval may continue to manufacture articles that meet the original TSO without obtaining a new acceptance, authorization, or approval but must comply with the requirements of this chapter.

From the FAA's page on TSO Cancellation/Withdrawal:

What is the difference between a "Canceled TSO" and "Withdrawal of a TSO authorization"?
A "Canceled TSO" means that the TSO has been rendered inactive and the FAA will no longer issue new TSO authorizations against that particular TSO. However, any existing TSO Authorizations obtained prior to the cancellation of the TSO are still valid and may continue to be manufactured and marked in accordance with that authorization.
The "Withdrawal of a TSO authorization" means that a specific authorization letter is withdrawn by the FAA and the approval to manufacture that particular article is terminated.
In summary, if the TSO is canceled, you can still make the article but you can't apply for a new authorization to that particular TSO revision level. If the TSO is withdrawn, you can't even manufacture the article under the TSO system (i.e. the FAA won't let you put a TSO label on it).

An example of the FAA suspending a TSO Authorization NavWorx ADS-B


text:
How does the UK declaration for FAA pilots work?
I came across http://publicapps.caa.co.uk/docs/33/SRG2140_Issue02_Enabled.pdf
This seems to suggest that an FAA PPL doesn't have to go through any exams / checkride / etc (other than the rudimentary "confirmation knowledge of Part FCL Air Law and ATC procedures") in order to be allowed to fly up to 28 calendar days per year in UK airspace.
Can anyone share additional details on how this process works, such as how long does it typically take to have the declaration verified, what costs (if any) are involved, whether this process has to be repeated every so often or is permanent, etc. ?

Word of advice - if you intend on doing it get your paperwork to the CAA well in advance (like 6-8 weeks at least) or pay for the over-the-counter service at Gatwick airport. The CAA are about the slowest organisation in the world.

It would be helpful if you would quote the relevant parts of the document.

@Jamiec thanks for the tip. Do you have some links or additional info on such over-the-counter service? Can I get the entire process done on the spot, same-day, by just getting to their office at Gatwick?

@GJ.https://www.caa.co.uk/General-aviation/Pilot-licences/Applications/Process/Gatwick-airport-over-the-counter-service/ its not walk-in, by appointment only. Be prepared to wait, even if you have an alotted time. CAA is a (bad) joke.

Having gone through this process, I thought I'd share my experiences and timescales:
Useful Docs:

https://publicapps.caa.co.uk/docs/33/InformationNotice2017029.pdf
http://publicapps.caa.co.uk/docs/33/SRG2140_Issue02_Enabled.pdf

1 Oct 2018
Get a checkout from an instructor and get them to sign the SRG2140 form and also certify copies of the docs required (log book, licence, medical etc, - details on SRG2140 form)
See https://www.caa.co.uk/General-aviation/Pilot-licences/Applications/Documentation/How-to-get-copies-of-your-documents-certified/ for details on how copies are certified i.e. the wording that must be used.
Scan and submit my SRG2140 form to the CAA via email to pilotvalidation@caa.co.uk
25 Oct 2018
Phone the CAA and talk to a very helpful person who says I need to

also send in SRG2142 and use fclweb@caa.co.uk
pay £45

So I filled in that form and sent it off by email again with my CAA reference number in the subject and to fclweb@faa.com and pilotvalidation@caa.co.uk
29 Oct 2018
Got a phone call (that I missed) and an email asking for my credit card details to pay the £45
I called them back and paid the money
Later that day I got another email confirming they had sent a request to the FAA and suggesting I chase the NAA:
We are contacting you to inform you that we have processed your application for the Verification of your Third Country ICAO licence, and a verification request has been sent to the FAA.
To help ensure that the verification is received in a timely manner, you may wish to contact the NAA directly. Additional requirements may need to be met in order to release your personal details to the UK CAA.
8 Nov
I discovered that the FAA has an online portal that allows you to request a validation report so I went to https://amsrvs.registry.faa.gov/amsrvs/main.asp and asked it to send a report to me and also directly to fclweb@caa.co.uk and pilotvalidation@caa.co.uk
12 Nov
Received the validation receipt:
As the current holder of an FAA licence with a validating medical certificate, you are entitled to exercise the private privileges of the ratings for which you are qualified on your FAA certificate in UK Airspace without remuneration.
Please note that your privileges on G-registered aircraft are restricted to day VFR regardless of the medical held, as stated in ORS no.1228: ORS4. FAA Certificate holders with a valid IR flying N-registered aircraft may continue to exercise IFR privileges in UK Airspace.
Should you be ramped checked, you will have to produce a copy of your declaration made on form SRG2140 and a copy of this email.
So now I can fly G-Reg aircraft in the UK.
In hindsight, if I had to do it all again I would:

submit both forms at the same time
once they had taken the money, gone on to the FAA website and request the validation be sent directly to the two CAA emails addresses

Next step is to do a conversion to an EASA licence which involves:

(a) pass a written examination in Air Law and Human Performance;
(b) pass the PPL, BPL or SPL skill test, as relevant, in accordance with
Part-FCL;
(c) fulfil the requirements for the issue of the relevant
class or type rating, in accordance with Subpart H;
(d) hold at least a Class 2 medical certificate, issued in accordance with
Part-Medical;
(e) demonstrate that he/she has acquired language proficiency in accordance with FCL.055;
(f) have completed at least 100 hours of flight time as a pilot.

As per https://eur-lex.europa.eu/legal-content/EN/TXT/?qid=1524553458669&uri=CELEX:02011R1178-20160408
ANNEX III – Part B “CONVERSION OF LICENCES”

Very nice first post, welcome to our site!

Thank you for this. Can you just add for how long do the declaration-based privileges last? And if not indefinitely (while the FAA license is valid), then what procedure should be done to extend it, and how often?

@GJ I'm not sure how long they last, IIRC my instructor said it was 12 or 24 months.  I am going for an EASA conversion so hopefully that will be in place before the FAA validation lapses


text:
Which direction of a forward slip is safer during a turn to final?
Suppose I'm making a turn from a left base to final and, realizing I need to kill quite a bit of excess altitude, decide to use a forward slip with full rudder deflection (along with full flaps).
Which direction of rudder deflection would be preferable?
I suppose that yawing left would result in less bank and therefore a lower load factor, which may be safe at low speed and low altitude. But yawing right, will result in a steeper bank which (under the same IAS) will result in an even higher rate of descent thereby getting me to the ideal glide slope earlier and leaving more time to fully stabilize before the threshold.

GJ, would you please elaborate on your comment of AOB being more or less in the different directions?  Because you control AOB...  Also, my first impulse is that it really doesn't matter, choose the direction that gives you best visibility and is suited to left/right handedness, but then I considered that precession, P-factor, slipstream, etc. may make one preferable over another, but I haven't heard anyone else mention it.  (only general cautions about slipping in a turn)  What factors should we consider?...

@MichaelHall sure, the idea is that the particular turn radius needed for aligning with the runway could be achieved either by a low AOB skid (yaw left in the example) or high AOB slip (yaw right in the example).

Ok, I understand now.  I would agree with the others then, don’t start your slip until out of the turn,

In hopes of being useful at the cost of being less precise- if you're turning base to final to the LEFT, the safest way to slip while turning is ROLL LEFT, RUDDER RIGHT, NOSE DOWN.

I use slipping turns all the time.
If you want to do a slipping turn you yaw the nose OUT of the turn, being careful to keep G loads low. It's NOT a dangerous maneuver if you keep speed up and nose down while doing it although you should use a bit more aggressive nose down than in a straight side slip to account for the slight extra G from the turn.  If you keep the wings largely unloaded and let the airplane fall semi-ballistically, you aren't going to stall.
To do it safely however, you do have to know how to fly the plane.

I agree.  In gliders we routinely practice slips the full length of the pattern, including turns, to simulate failed spoilers.  In fact I was required to do this during my FAA practical test.   Yawing away from the runway ensures you are always banked toward the runway (straight and turning).  It's exciting - noisy, shaking, airspeed reads zero, sight lines changed - but I tell myself "the wings don't know the plane is slipping".  Just be sure to keep the airspeed up.  Needless to say, you learn this at altitude then do the pattern with an instructor.

Don't do either until you are on final. Otherwise you can get into an uncoordinated mess and possibly stall, or worse snap roll the airplane, and without sufficient altitude to recover, you're hitting the ground hard.  Possibly fatally.
Once on final it depends on where the wind is coming from. Straight ahead or from the left, I like dropping left wing & adding right rudder so I can see better.
Wind from the right, you need to drop the right wing and add left rudder.
Try your suggestion 2000-3000 ft above ground level first.  Go find a road to follow as a runway and see what the plane does. If it simply stalls, we've been trained as Private Pilots how to recover.  Something more severe, ask a local instructor what to do.

Totally agree.   Turns in the pattern need to be fully coordinated and "flying straight".  Only after you've completed the turn, gone wings-level, and stabilized can you properly judge what maneuver to do, to what degree, and execute it safely.

Yes, but also not necessarily. There is nothing dangerous about a properly executed turning slip, and this is especially common with older flapless planes like Cubs. But this is useful advice for student pilots, who can peel back the nuance later in their training.

Slipping turns are not inherently dangerous.  NEVER do a skidding turn, with nose yawed in same direction as bank (i.e. pilot is holding low-side rudder), anywhere near the ground.  It is an invitation to a stall-spin accident.
If you want to wait till you are beginning final approach to begin the slip, AND wind is not a factor, it makes sense to keep some of the bank that you had from your base-to-final turn and then cancel the turn rate with opposite rudder, i.e. top rudder.  Obviously, you can't do a non-turning slip by applying rudder in the same direction as bank.

Rudder opposite the bank always.  Maintain a safe airspeed for the conditions. Be familiar with the airplanes handling.
If you need to know your safety margin, hire an instructor and ask for training at a high altitude that would allow for spin recovery in an aircraft approved for spins to see at what airspeed the plane will stall in a full turning sidedlip.
Ensure your real, pattern altitude, turning slips are done well above that speed.

"Supposing I'm making a turn from left base to final, and realizing I need to kill quite a bit of excess altitude" 4th Edition
Pushing the rudder into the turn is not an option due to stall/spin risk.
An aileron slip while turning is possible.  To execute the maneuver, one would carefully increase roll to the left (left pattern) without increasing load on elevator, resulting in an increased rate of descent. The inside aileron goes up more, the outside down.  Your approach speed is kept at a safe margin over stall.
What could go wrong??  If you practiced a simulated approach well above AGL, you would know!
The slip maneuver is safer for aircraft with low wing loading and ample tail volume as the tail will naturally lower the nose with increased VV and the wings will help reduce VV when you roll out.  With practice, you do these all day long in a glider.
If you are flying a "brick", VV can get out of hand and high loaded wings will be much more prone to stalling if care is not taken to monitor VV, limit slip angle, and unload or even pitch down elevator while rolling out.
In all cases, the "round out" should start at a higher altitude due to the higher angle of descent (power on low angle landing can be "greased in" with hardly any round out at all).
Try it first at altitude.  Practice makes perfect,


text:
What is the most fuel efficient climb speed for a C172 SP?
Assuming ISA, sea level, no wind, max gross weight, what speed would be most fuel efficient per 1,000 ft of climb?
Alternatively, if that converges to the most fuel efficient cruise speed, when there's no time and/or distance limitation for completing such a climb, please explain why this is so.

The purpose in climbing is to get there so you can get on with the rest of the flight. What practical purpose could possibly be served by optimizing fuel in a climb? Especially at max weight??

@DouglasHeld Several scenarios, in my case. One is the need to climb to a certain altitude ahead of increased underlying terrain elevation or cloud tops in VFR. If such an obstruction is, say 50nm ahead, it makes no difference if I make the climb over 2 miles or 40 miles, and the important thing for me is to minimize fuel consumption.

Actually it does, and GJ. comment below tells why.  Range in ALL aircraft improves with altitude.  This is why one climbs at Vy and then cruises more efficiently at a higher altitude.  Slow climb does not take advantage of greater GROUND SPEED as well. KTAS formula shows this by factoring in pressures.  I would try both and compare, nothing wrong with slow climb if you enjoy it.  At reduced power settings, I would keep carb icing in mind.

First consider the four forces on the airplane in the cruise:
          ⇧
        Lift
⇦ Drag    .  Thrust ⇨
       Gravity
          ⇩

Drag is a function of the airframe, attitude, and airspeed. If the airplane can fly, then drag is more then offset by thrust. The lift is generated by a component of the forward motion when the flying surfaces convert the horizontal force to vertical force. And finally, the lift force is equal to the force exerted by gravity.
Now let's talk about fuel efficiency. That is how much you can do with a quantity of fuel, and you didn't specify whether you're interested in fuel for time (endurance) or fuel for distance, or something else. But let's discuss the major components of fuel usage:

Engine motion. The engine is more efficient in terms of power at its higher rated power setting. So, you would think efficiency would be gained by selecting a higher power (assuming you could also climb with less than 100% power).
Heat. I guess a typical heat engine like a piston engine consumes about 50% of fuel energy to heat (rough guess, it's prob. in the range 35% - 65%). But the heat engine is more powerful the greater the difference between inlet temperature and exhaust temperature. So, the greater time you can spend at higher altitude is probably more efficient in fuel for power.
Cooling. Sometimes you need extra unburnt fuel for engine cooling. For example during climb, when the engine is at higher power and the airflow is reduced.

Now let's discus climbing. During a climb the aircraft has (in theory) to produce all the power necessary for level flight, plus a little extra for the change in elevation. I would posit, without any evidence, that the fuel spent on the elevation change is insignificant compared to the fuel required to stay aloft. (I will gladly accept a reference or metrics if someone has them). But think about it, if you put the aircraft in an elevator powered by a similar motor, it would take a few drops of fuel to raise a few feet. That's a thousand drops for a thousand feet - it is nothing versus constantly offsetting the the weight constantly as it does in flight.
So what is the more fuel efficient climb procedure? It is the technique using:

Higher engine power
Maximizing time at higher altitude; minimizing time at lower altitude
Balancing the mixture to keep the engine as hot as its design will allow. (you said fuel-efficient. You didn't say cost-efficient. Don't violate your POH!)

In summary, to expend the least fuel, climb as early as possible, as quickly as possible (e.g. Vy) and with minimum acceptable richness.
And as long as we are talking efficiency, please also run the tanks dry at your final cruise altitude. This will maximize the most fuel efficient part of the cruise, when you are carrying less fuel.

A lightly loaded Cessna 172 needs 30hp to climb at 500fpm which is not insignificant. Also, your guess for engine efficiency is way off - 35% efficiency is actually typical.

I couldn't remember offhand if it is 40% efficiency or 40% wasted. I thought instead of looking it up I would use half.

It seems to me that whatever cruise speed was chosen, the most fuel efficient climb given that baseline fuel consumption in cruise, would be to keep the same power setting and climb in the slowest rate acceptable.
E.g. if cruising at 90 kts and a climb of 3,000 ft should be completed within at most 30 NM, then climb at 150 fpm.
This should result in about the same fuel consumption per time relative to staying at that same cruise speed and only slightly more consumption per distance since the ground speed will be minimally reduced (and even that may be compensated for by the increased TAS under such power setting due to the increase in altitude).
If the baseline cruise speed was chosen to be the max endurance speed, then such a climb should also be more or less the all round most fuel efficient.

I hope you get good results.  As a final thought, climbing slowly through airspace that has even and odd direction of west/east traffic may introduce a safety issue. This is why "jetliner style" Vy climb to cruise may be preferred.  But your inputs and the "Carson Cruise" question are appreciated.


text:
Can hood time in a glider be logged as simulated instrument time for FAA IR purposes?
I need to build my simulated instrument time toward IR. Can flying under the hood in a glider (with an instructor/safety pilot...) be legally counted as simulated instrument time in the same way would be the case had it been an ASEL?

Curious as to what country you live in, and what instruments are in the glider.  My understanding is that actual cloud flying in gliders is still fairly common in Great Britain, even with no attitude indicator.  (So, "partial panel"-- turn rate indicator as only gyro instrument.)  This sort of thing used to be widely done all over Europe.  Don't forget to bring your parachute!

Oops never mind you said FAA.  Still curious about what instruments are in glider.

In the US, yes. According to the FARs you only need to have three hours in an airplane in preparation for the flight test and you must fly the cross country in an airplane so that’s probably another 3 hours.
It looks like you can log hours under the hood in a glider, but it would have to be with an airplane CFII. However, I don’t know that you would be able to train in any of the areas of operation except for (4) Flight by reference to instruments; so it probably wouldn’t be worth your time. I don’t know how you would fly an approach or fly a course by reference to navigation systems.
So as a practical matter, the answer is no.

§61.65   Instrument rating requirements.
(c) Flight proficiency. A person who applies for an instrument rating
  must receive and log training from an authorized instructor in an
  aircraft, or in a full flight simulator or flight training device, in accordance with paragraph (g) of this section, that includes the
  following areas of operation:
(1) Preflight preparation;
(2) Preflight procedures;
(3) Air traffic control clearances and procedures;
(4) Flight by reference to instruments;
(5) Navigation systems;
(6) Instrument approach procedures;
(7) Emergency operations; and
(8) Postflight procedures.

As an aside, I used to rent out my Cherokee to helicopter commercial students who needed instrument training

(i) Five hours on the control and maneuvering of a helicopter solely
  by reference to instruments using a view-limiting device including
  attitude instrument flying, partial panel skills, recovery from
  unusual flight attitudes, and intercepting and tracking navigational
  systems. This aeronautical experience may be performed in an aircraft,
  full flight simulator, flight training device, or an aviation training
  device;

Actually, in my case I became quite proficient in nearly all of the required skills by simply using a flight simulator with simulated ATC (pilotedge) so that I mostly need to build the time just as a formality.

@GJ Agreeed. Simulators are great for practicing your scan, learning to multitask, and reading approaches. I have way more sim time than hood time.


text:
What do the partially filled circles on skyvector mean?
What does a partially filled weather status circle signify on Skyvector.com ?

It's an indication of how much of the sky is under that weather condition: IMC, marginal VMC, VMC.

what does a half-filled green circle mean then? half of the sky is VFR and the other half is what?

Not 100% sure.  I was looking at the weather reports that are pulled up when you hover over a circle, and the reports don't look that different between 1/4 green, 1/2 green, 3/4 green.   Could be how much of the area is under a high ceiling. So VMC, but not fully clear skies.

Actually the colour indicates condition (green = VMC, blue = Marginal VMC, red = IMC) and the fraction is cloud coverage (empty = clear, full = overcast, fractions in between). It is taken from https://www.1800wxbrief.com/.

@GJ., it means VFR (green), partly cloudy (1/2).

1/4-full circle would mean 1/4 clouds, or 3/4 clouds?

@gj If you are referring to the circle an the chart above Flying Wing, it is 3/4 clouds.


text:
What does "Procedure NA" mean, when applied to a VOR sector?
In the RNAV-A IAP for KGAI there's a note "Procedure NA for arrival on EMI VORTAC airway radials 151 CW 269". It appears that arriving from any airway radial to EMI would have the NoPT (of the 205 outbound) apply to it, and it's not clear how to interpret this note when joining the approach at WOOLY as the IAF. So what exactly does this note mean?

"NA" means "not authorized" here, because they don’t want you making a 163° turn from V265 NB, a 127° turn from V268 NB or a 115° turn from V3-166-457 EB over EMI. If you’re arriving from outside that arc, the NoPT routing applies.
A note like this is standard any time there is an airway crossing an IAF or feeder fix, because they don’t want you doing an uncharted course reversal over a VOR or anticipating the turn so early that it goes outside the protected airspace.
The note says nothing about restrictions on using WOOLY as the IAF, e.g. if you are arriving on V44-214. There’s a hold charted there, so using that as a course reversal is fine if you need one.

So if I'm arriving on V265 NB, how am I supposed to enter and perform the approach? (assuming lost comm / no radar vectors)

@GJ. In that case, file V265.YANNI.V44.WOOLY..KGAI There’s no reason to go all the way up to EMI.

Sure, Stephen, but the question is about whether, when passing WOOLY in such a way, you must or must not perform the PT.

@GJ. WOOLY is only marked NoPT if you arrive from EMI. You could ask ATC to clear you for the straight-in, but odds are they’re going to vector you to join final anyway, which is NoPT by rule.

NA is "not authorized" here - other charts write that out.
This could be due to some restriced airspace to the south - R151 to R269 from EMI - Washington DC does have some restricted airspace in that region.

Not sure I get it. For which arrivals at WOOLY do you take it that a PT is required, and in which (other than the 205 radial from EMI) is the PT NA?

If you arrive at EMI outside the specified radials, you fly the NoPT routing to WOOLY and keep going. If you arrive at EMI on one of the specified radials, the procedure is Not Authorized. If you arrive at WOOLY from something besides the NoPT track from EMI, then you fly the holding pattern (a.k.a. Holding in lieu of a Procedure Turn) to align yourself, and then go on to the FAF.

@RalphJ what's the logic in that? E.g. if I arrive in WOOLY already on a course of 220, better aligned than from EMI, and already at the exact altitude I should be, and still have to make a full lap in the hold?

@GJ.  In a case like that, you could request ATC clearance to proceed straight in & skip the hold. There's nothing different between this approach & any other Holding-in-lieu-of procedure in that regard.


text:
Does a previous AFSP approval get invalidated when making a new training request?
If I've already been approved, and started training in one school and then create a training request to switch to another school, does the previous approval expire?
In other words, can I be simultaneously approved on two different schools?
I'm considering transferring temporarily to continue flying in a different region, but would like to have the option to return to my original school without yet another training request...

Yes, you can have multiple requests open at the same time. This is from the AFSP Candidate Application Guide (step 7):

It is possible for a Candidate to have several active training requests at a given time. These requests may be for the same or different flight training providers. Each training request form will be processed separately; AFSP approval is valid only for the Provider listed in the application.

I was actually asking specifically about having an approved status on more than one request, which is slightly different than having multiple requests merely open at the same time. Are you sure a previously approved request doesn't get deleted or become invalid when a new one gets approved?

@GJ Yes, that's what the second sentence is saying: each application is handled independently. I've had two open myself in the past, some 'good' news is that you only have to submit your fingerprints once and they can use them for all future applications.


text:
Payload capacity of a quadcopter/helicopter with increasing altitude
I am trying to create a simplified model of a heavy-lift drone payload at different altitude during hover mode. I tried to derive it simply by considering that the thrust, $T$ is equal to the weight of the drone, $G$ and a specified payload, $L$ which is given by the following equation:
$$ n_r \cdot T = G + L $$
From this formula, we know that the thrust at different altitude will vary, and this relationship is captured by the following equation:
$$ T = C_T \cdot \rho \cdot (N)^2 \cdot D_p^4 $$
$$ \rho = (P_a/P) \cdot \rho_0$$
$$ P_a = P \cdot (1 - 0.0065 \cdot \frac{h}{273 + T})^{5.2561}$$
where h is the altitude (m), $T$ is the temperature at sea level, $P_a$ is the local pressure at the given altitude (Pa), $P$ is the pressure at the sea-level, $ \rho $ is the local density at an altitude, $\rho_0$ is the standard air density at sea-level. From this equation, we can simplify it to be the following:
$$T(\rho, N, D_p)$$
$$\rho(P_a)$$
$$P_a(h, T_t)$$
where the relationship can be described by
$$P_a \propto \frac{1}{h}$$
$$ \rho \propto P_a$$
$$T \propto \rho $$
From the relationship above, we can clearly see the connection between the altitude and thrust. In the case of increasing altitude, the local atmospheric pressure decreases, which leads to a decrease in local density. This in turn, leads to a decrease in thrust.
Re-arranging the first equation to give:
$$ L = n_r \cdot T - G $$
Plotting this relationship with a constant propeller speed, constant weight of drone (300kg/3000N) and a constant thrust coefficient, I manage to get the following plot:

Analyzing the plot, from my understanding, as the altitude increases, the payload that the drone can carry decreases. However, what I am confused with is that, in the formula, I only used the weight of the drone without the payload, and the plot tells me the payload capacity of the drone at different altitude. However, if this payload is then placed onto the drone, the plot would have changed as the weight of the drone then changes.
Is there anything that I am missing here?
I then went on to read some literature on helicopter performance and I manage to find this:

The plot looks similar to my plot to some extent, but I've been trying to search for equations that describe this performance for helicopter so that I can compare my derivation to it. Can anyone point me in the right direction?

In helicopter momentum theory, thrust is usually written as $$T = C_T \cdot \rho A (\Omega R)^2$$ And if we keep $C_T, A$ and $\Omega R$ constant, we can see that thrust decreases linearly with decreasing density. Which is basically what hes been depicted in the OP plots.
However, in the hover the thrust must be equal to the weight at a variety of altitudes. Usually this is done by varying $C_T$ - increase collective, and the helicopter rises to a higher equilibrium altitude. Or increase $\Omega$ of a quadcopter.
The altitude limit at a given total weight would be a function of available power, as further detailed in this answer. The IGE hover ceiling graph of the Schweizer 296 depicts the limits imposed by the engine power limit at varying altitudes.


text:
Would an Autogryro/Gyrocopter with wings be safer than a normal autogyro in a negative G situation?
Typically Autogyros experiencing "negative Gs" will crash.
I wonder if an Autogyro with wings would work as a fail safe for this situation? Would it allow the pilot to recover assuming this have enough height stall?

Related: https://aviation.stackexchange.com/questions/49865/why-does-xkcd-say-that-autogyros-will-crash-if-the-pilot-reacts-to-a-stall-as-in

It would only work if the wings provided enough lift to allow a controlled descent after the rotors had stopped.

Honestly, it looks like just two more things for the rotor blades to collide with.

An autogyro with wing is an airplane with an useless rotor on top of it

Maybe. A big issue with a negative-g situation in an autogyro is loss of longitudinal control caused by the loss of drag on the rotor. The rotor drag provides a lot of nose-up moment, and the loss of it can cause the autogyro to flip. This makes recovery essentially impossible.
Wings positioned near the center of gravity provide relatively small longitudinal moments. They may help (if they are mounted forward of the center of gravity and so produce nose-up moment), but it is unlikely to be enough. A tailplane, on the other hand, might give enough nose-up moment to keep the autogyro from flipping over. The wings could also be helpful for roll control, especially if they are equipped with ailerons.
With sufficient altitude, if the autogyro can be kept under control it should be possible to get the rotor RPM back up and resume normal flight.

Given sufficient altitude, a stalled autogyro should be able to enter autoration and land safely in the same way as a helicopter.

After a negative-G incident, the rotor rarely stays in one piece...


text:
Precautions for LauncherOne off centerline drop
Virgin Orbit's LauncherOne is carried in the spare engine position of its 747 carrier aircraft inboard from the inner engine on the portside wing. At launch, the orbital launch vehicle is dropped from the aircraft in midair.
According to Wikipedia, LauncherOne is approximately 30 tonnes. The actual weight could be less, but in any case that seems like a lot of weight off centerline to drop in midair.
To me that looks like it could be a problem, but it is clearly feasible since it has been done already.
What sort of precautions are are needed to do this safely? Is it just a matter of leisurely adjusting aileron trim after launch, or is some special preparation needed for this specific issue?

It's worth noting that the aircraft is not flying level at the point of release. The aircraft does a "zoom climb" maneuver and is pitched steeply nose-up at the moment of release. This is going to dramatically reduce the roll.
Public videos of the launches are hard to find but they are out there... The 747 does roll right somewhat, but in an aircraft with a takeoff weight around 900,000 pounds or 400+kg, there's going to be plenty of control surface authority to keep things from going crazy when 60,000 pounds are released from the inboard portion of the wing.


text:
What are the exit temperatures of bypass flow
Consider a typical high bypass turbofan designed for cruise at approximately mach 0.8, with a separate nozzle for the bypass air.
Ideally, the bypass air would be compressed adiabatically by the intake and the fan, and then expanded adiabatically in the nozzle to ambient pressure and temperature.
But there are losses in the intake, fan and nozzle, all of which generate a little bit of heat.1 I also expect some heat is absorbed from the engine core.
Moreover, according to this answer, the bypass air exiting the nozzle in cruise is typically under-expanded, which might raise the temperature at the nozzle exit some more. The aforementioned answer also states that the bypass air is exiting near the speed of sound.
As this answer explains, the core airflow can get significantly faster than the speed of sound in ambient air and stay subsonic; the speed of sound is higher in the exiting air because of its higher temperature.
This makes me wonder, what is the temperature difference between the bypass air exiting the nozzle and the ambient air? In particular, is it enough to raise the speed of sound in the exiting air a meaningful amount?

1: The kinetic energy at 300 m/s is 3002/2=45000 j/kg. The specific heat of atmosphere is around 1 kj/(kg*K), so even 10% losses would only raise the temperature 4-5K. Hence little bit.

The main reason temperature might change is heat exchange with the core flow. Otherwise, bypass flow just isn't being compressed enough to notice.

@Therac : Sorry for the late response. Do you have any estimates on heat transfer from the core?

No concrete estimates. I would guess the temperature rise to be pretty small,  due to large volume of the bypass flow. It can be calculated though. You can ask it as a separate question: how much heat passes from the core to the bypass flow.

I've attached a simple Matlab program that does a cycle analysis for a bypass flow.
With M0=0.8, fan efficiency of 0.9, inlet and nozzle pressure ratios of 0.99, and a fan pressure ratio of 1.15, the ratios of speed of sound at exit to inlet is 1.0051.
So, there is no significant change.
clear all
format compact

% Simple non-ideal 'cycle' analysis for a shaft-driven ducted fan.
% Cycle is in quotes because no work balance is required for this
% calculation.  The shaft power is assumed delivered on demand.  Component
% losses are included as appropriate for this model.

% Since this is a point-calculation, this engine does not need to be sized.
% Thrust and shaft work are calculated per mass flow.

% The nozzle is assumed to expand the flow to ambient pressure - or choked
% flow, whichever happens first.  The areas required to make this happen
% are not actually calculated.

%              ----------------------------
%          ---/    | \                     \----
%        -/        | \                          \
%                  |  \----------
%                  |   \==MMM    \
%  -- . -- . -- . -- . -- . -- . -- . -- . -- . -- . --
%   0             2     3                7       8/9

% Station numbering: (provisions for variable area are just that)
% 0   -- Freestream
% 2   -- Diffuser exit/fan front face
% 3   -- Fan exit
% 7   -- Nozzle start (fixed area)
% 8/9 -- Nozzle throat/exit (variable area) converging-only nozzle

gamma = 1.4;               % --
R = 1716.59;               % ft lbf/(R slug)
cp = R*gamma/(gamma-1.0);  % ft lbf/(R slug)

T0 = 59+459.67;            % Rankine
P0 = 2116.22;              % psf
M0 = 0.8;                  % --

% Inlet pressure ratio.  Adiabatic inlet.
IPR = 0.99;

% Fan efficiency.  Adiabatic fan.
Feta = 0.9;

% Nozzle pressure ratio.  Adiabatic nozzle.
NPR = 0.99;

a0 = sqrt(gamma*R*T0);
u0 = M0*a0;

Pt0 = P0*(1.0 + ((gamma-1.0)/2.0)*M0^2.0)^(gamma/(gamma-1.0));
Tt0 = T0*(1.0 + ((gamma-1.0)/2.0)*M0^2.0);

FPR = 1.15;
FTRi = FPR^((gamma-1.0)/gamma); % Ideal FTR
FTR = 1.0+(FTRi-1.0)/Feta;

Pt2 = Pt0 * IPR;
Tt2 = Tt0;

Pt3 = Pt2 * FPR;
Tt3 = Tt2 * FTR;

Pshaft = cp*(Tt3-Tt2);       % Shaft power per mass flow

Pt9 = Pt3 * NPR;
Tt9 = Tt3;

% Check for choked flow at nozzle
P9choked = Pt9 /(((gamma+1.0)/2.0)^(gamma/(gamma-1.0)));

if(P9choked > P0)
  disp('Choked');
  P9 = P9choked;
  M9 = 1.0;
  T9 = Tt9 /((gamma+1.0)/2.0);
else
  disp('Fully expanded');
  P9 = P0;
  M9 = sqrt((((Pt9/P9)^((gamma-1.0)/gamma))-1.0)*(2.0/(gamma-1.0)));
  T9 = Tt9 / (1.0 + ((gamma-1.0)/2.0)*M9^2.0);
end

a9 = sqrt(gamma*R*T9);

u9 = a9*M9;

rho9 = P9/(R*T9);

f = u9-u0+(P9-P0)/(u9*rho9);  % Thrust per mass flow

pprop = f*u0;              % Propulsive power per mass flow

etao = pprop/Pshaft;  % Overall efficiency

etap = 2*u0/(u9+u0);  % Classical propulsive efficiency

TSEC = f/Pshaft;             % Thrust specific energy consumption  (s/ft)

I recently accepted this answer after long delay, sorry about that. Seems to me you show that inlet-, nozzle- and fan-losses have negligible effect on exit temperature.  I just want to comment that this program doesn't take into account heat transfer from the core. According to a comment to the question this has greater potential to increase temperature than losses in compression and expansion, but perhaps estimating that is not a reasonable question.

The program above was designed to model an electric motor driven fan, so no core at all.  I do not see any evidence that the heat transfer from the engine core and core flow is a substantial contribution to the bypass flow.  If I was modeling a traditional turbofan engine, I would still most likely ignore core-to-bypass heat transfer.  Modern bypass ratios of 10 really emphasize how much flow goes through the duct.


text:
Why can APU supply both bleed air and electrical power up to 10.000 feet but bleed air alone up to 17000?
I am reading my company's FCOM and I saw there the following information.
APU Bleed + Electrical Load: Max Alt 10000 ft.
APU Bleed: Max Alt 17000 ft.
APU Electrical Load: Max Alt 41000 ft.
If APU can supply bleed air up to 17000 ft, why supplying electrical load at the same time is not possible?
Does bleed air usage on the APU affect the generation of electrical power?
Thank you all for your answers.

It's a function of the relative demand each service puts on the APU.  You have to remember that bleed is power consumed; you're just taking it in a different form.
As a turboshaft jet engine, the primary limitation on total power is the amount of heat the turbine can take.  When you take bleed from the compressor, the turbine is forced to generate a lot of torque to drive the compressor but it's not getting the mass flow downstream through the burner can and turbine that it would get without any bleed (it's like you were cranking a fan by hand to cool yourself, but some joker put a piece of cardboard between you and the fan to divert most of the air - you're getting some air but you'd be pretty hot and bothered and you just can't do the total work you could do if you were getting all the air).
Since one of its primary functions is to provide hot air to the cabin as opposed to making thrust, a much higher proportion of its mass flow is bled off an APU's compressor compared to a main engine, and is a significant portion of the engine's energy potential. The generator, on the other hand, puts much less demand on the engine.
The result is for the same shaft HP available to drive the generator, the turbine temperatures are much higher when bleed is on, basically because it's doing the job of providing shaft HP and providing compressed air. Hence the 10000 ft limitation when doing both at the same time.
The other altitude limits are based on the effect each service individually has on the APU's temperature limits.  The compressed air service, the bleed, uses most of the APU's power potential and maxes out at 17000 ft, with the generator a distant second and when it's the only service the APU has the HP available to run the generator all the way to 41000 ft.